The term "conversion coating" is a well known term of the art and refers to the replacement of native oxide on the surface of a metal by the controlled chemical formation of a film. Oxides or phosphates are common conversion coatings. Conversion coatings are used on metals such as aluminium, iron, zinc, cadmium or magnesium and their alloys, and provide a key for paint adhesion and/or corrosion protection of the substrate metal. Accordingly, conversion coatings find application in such areas as the aerospace, architectural and building industries.
Known methods for applying conversion coatings to metal surfaces include treatment with chromate or phosphate solutions, or mixtures thereof. However, in recent years it has been recognised that the hexavalent chromium ion, Cr.sup.6+, is a serious environmental and health hazard. Phosphate ions can also be detrimental, particularly when they find their way into natural waterways and cause algal blooms. Consequently, strict restrictions have been placed on industrial processes and limitations have been placed on the release of such solutions to the environment. This leads to costly effluent processing.
In the search for alternative, less toxic conversion coatings, research has been conducted on conversion coatings based on rare earth compounds. One prior conversion coating process has been described in Australian patent specification AU-A-14858/88 which is incorporated herein by reference. That conversion coating process comprises contacting a metal surface with a solution formed by an aqueous acidic solution containing cerium and H.sub.2 O.sub.2 in which some or all of the cerium has been oxidised to the +4 valence state. It is asserted in AU-A-14858/88 that an increase in the solution pH in the region of the metal surface to a sufficiently high value causes precipitation of a cerium containing coating on the metal surface.
There is, however, considerable room for improvement in the properties of prior rare earth element based conversion coatings, such as adhesion, and in the time required to deposit those coatings. The need for improvement is particularly true for conversion coatings on certain metal alloys, such as 3000, 5000 and 6000 series aluminium alloys, which coatings can be slow to deposit and have variable adherence or no adherence.
Accordingly, it is an object of the present invention to provide a process and solution for forming a conversion coating on a metal surface which overcome, or at least alleviate, one or more of the disadvantages or deficiencies of the prior art. It is also an object of the present invention to provide conversion coated metal surface formed by the process of the invention.
It has been discovered that addition of one or more additives, having particular compositions, to the coating solution can assist in accelerating the coating process and/or improving adhesion of the conversion coating to the metal surface.
Throughout the specification, reference will be to the CAS version of the Periodic Table, as defined in (for example) Chemical and Engineering News, 63(5), 27, 1985. Furthermore, as used herein, the term "transition elements" or "transition metals" refers to the elements of the Periodic Table from scandium to zinc inclusively, yttrium to cadmium inclusively and lanthanum to mercury inclusively. Moreover, as used herein, the term "rare earth" elements, metals or cations refer to the elements of the Lanthanide series, namely those having the atomic number 57 to 71 (La to Lu), plus scandium and yttrium. In addition, the term "higher valence state" means a valence state above zero valency.